Selective CO2 electrolysis to CO using isolated antimony alloyed copper

Renewable electricity-powered CO evolution from CO2 emissions is a promising first step in the sustainable production of commodity chemicals, but performing electrochemical CO2 reduction economically at scale is challenging since only noble metals, for example, gold and silver, have shown high performance for CO2-to-CO. Cu is a potential catalyst to achieve CO2 reduction to CO at the industrial scale, but the C-C coupling process on Cu significantly depletes CO* intermediates, thus limiting the CO evolution rate and producing many hydrocarbon and oxygenate mixtures. Herein, we tune the CO selectivity of Cu by alloying a second metal Sb into Cu, and report an antimony-copper single-atom alloy catalyst (Sb1Cu) of isolated Sb-Cu interfaces that catalyzes the efficient conversion of CO2-to-CO with a Faradaic efficiency over 95%. The partial current density reaches 452 mA cm−2 with approximately 91% CO Faradaic efficiency, and negligible C2+ products are observed. In situ spectroscopic measurements and theoretical simulations reason that the atomic Sb-Cu interface in Cu promotes CO2 adsorption/activation and weakens the binding strength of CO*, which ends up with enhanced CO selectivity and production rates.

coordination structure of the catalyst changes significantly after reaction.

Response
We thank the reviewer for his/her important suggestion. We have provided the fitting results of Sb K-edge EXAFS in our original supplementary information (Supplementary

Comment 2
In the reviewer's opinion, the current stability data is not enough to support the conclusion regarding the catalyst with impressive performance rather than performing economically at Phone: +86 028-61838256 • E-mail: chuan.xia@uestc.edu.cn • Web: https://www.chuan-lab.com/ scale. It is strongly recommended to extend the duration of stability test and further explore the structure evolution of catalysts (such as the coordination structure of Sb).

Response
We thank the reviewer for this important suggestion. In our case, the stability test in MEA lasted 10 hours due to flooding and carbonation problems in the gas diffusion electrode (GDE) rather than the degeneration of the catalysts. After long-term CO2 electrolysis, the loss of hydrophobicity will damage the triple-phase boundary (where CO2 reduction mainly occurs), obstructing the diffusion pathways for CO2 and further leading to a sharp decrease in the CO2RR activity [ChemSusChem 13, 400-411 (2020)]. In addition, during CO2 reduction, basification due to hydroxide formation led to the conversion of CO2 to carbonate, which caused precipitation of salts on the GDE [ChemElectroChem 6, 5596-5602 (2019)].
This phenomenon will lead to blocking of diffusion passage and greatly influence the stability of the CO2 reduction reactor (Fig. R1). To confirm the intrinsic stability of the Sb1Cu-5 catalysts, we used an H-cell as the reaction reactor to exclude the factor of an unstable triple-phase boundary. The long-term stability test showed that FECO remained at approximately 90% throughout 100 hours of continuous electrolysis at a current density of -10 mA cm -2 , and the cathode potential was quite stable without obvious fluctuation ( Supplementary Fig. 22, copied below). In addition, ex situ Sb K-edge EXAFS of the catalyst after 100 hours of CO2 electrolysis demonstrated intact isolated Sb-Cu interfaces ( Supplementary Fig. 18, copied below). In summary, the Sb-Cu atomic sites and CO2-to-CO performance were robust during long-term CO2RR, indicating potential industrial application of the Sb1Cu catalysts.

Comment 3
Expect for the activity, the selectivity of Cu was also modulated by the introduction of Sb.
Thus, the discussion about catalytic performance should be modified.

Response
We appreciate the reviewer for his/her good suggestion. In the CO2RR performance section, we updated our description to "This leads us to believe that the intrinsic activity and selectivity of Cu was significantly modulated by the introduction of isolated Sb-Cu atomic interfaces.", to further highlight our modification in both activity and selectivity. In addition, we have strengthened our modification in selectivity compared with pure Cu in the abstract and conclusions sections. In the abstract section, we mentioned "…… the C-C coupling process on Cu significantly depletes CO* intermediates, …… producing many hydrocarbon  R2). In the CO2RR performance test, the Cu-Sb random alloy showed FECO <50% (e.g., ca. 45% at -100 mA cm -2 ) ( Fig. R3), much lower than that in Sb1Cu-5 (ca. 95% of FECO at -100 mA cm -2 ). Certain amount of hydrogen and formate were also found in the CO2RR products on the Cu-Sb random alloy. Considering the few or even lack of isolated Sb-Cu interfaces in the Cu-Sb random alloy, the better CO2RR performance on the Sb1Cu-5 SAA than the Cu-Sb random alloy was presumably ascribe to the isolated Sb-Cu atomic interfaces. Therefore, this result further verified the importance of the isolated distributed Sb-Cu atomic interface.

Response
We thank the reviewer for raising this question. Mass transfer is an important factor in CO2RR. To explore the factor of mass transfer in CO2RR performance, we conducted the CO2RR test in an H-cell and a flow cell with different mass transfer efficiency. In an H-cell, CO2 gas is purged into the aqueous catholyte, and the dissolved CO2 molecules are adsorbed on the electrocatalyst surfaces and undergo reduction (Fig. R4a). Achieving high rates of reaction in these conditions is limited given the low concentration (33 mM) and slow diffusion of aqueous CO2 (diffusion coefficient tCO2 = 0.00176 mm 2 s -1 at 20℃) [Nat. Energy 7, 130-143 (2022)]. In the H-cell, the FECO on the Sb1Cu-5 catalyst maintained >80% at low current densities, and dramatically decreased at more negative potential due to the mass transport limitation that favored HER. In contrast, in a flow cell, the CO2 gas diffuses through the back of the GDE-based cathode (Fig. R4b), where gaseous CO2 is fed directly to an interface between the catalyst and electrolyte [Chem. Soc. Rev. 50, 12897-12914 (2021)].
This facilitates the rapid mass transport of CO2 to the catalyst surface, where it is bound and subjected to the proton and electron transfers necessary to form a given product. By taking advantage of flow cell, remarkably high CO partial current density was achieved for the Sb1Cu-5 catalyst while maintaining a high CO selectivity above 90% (Fig. R5)

Comment 6
It is suggested to give a radar chart when comparing with the previously reported state-ofthe-art CO-selective electrocatalysts, which would be much easier to show the advantage.

Response
We appreciate the reviewer for his/her valuable suggestion. We made a radar chart to compare our catalyst with previously reported state-of-the-art CO-selectivity catalysts and  Fig. 23, copied below). Sb1Cu-5 catalyst exhibited a much higher CO current density, CO Faradaic efficiency and CO2 conversion rate than the previously reported CO-selective catalysts.

Comment 7
The experimental section needs to be far more detailed. It's far too limited for the work to be reproduced effectively. Please can the authors also include a chemicals (source and purity) section.

Response
We have updated the experimental section with more details and included the source and purity of the chemicals (coped below).

Comment 8
Concerning the CO2-TPD results, whether the desorption peak of could be directly attributed to CO2? Since the pretreatment temperature is only up to 400 o C, the peak around 400-500 o C needs to be determined in combination with MS results.

Response
We appreciate the reviewer for this very important suggestion. We repeated the CO2-TPD tests and added an MS sensor to verify that the peak at approximately 400℃ was attributed to CO2 desorption (Fig. R6). The MS signal showed the same peak at approximately 400℃, which matched well with the TCD results, confirming that the peak around 400℃ was attributed to CO2 desorption. However, we apologize for ignoring the possible reconstruction of Sb1Cu-5 under high temperature. The XRD pattern showed peaks of Sb2O3 in Sb1Cu-5 after treatment at 400℃, demonstrating phase separation of Sb and Cu metal in the Cu-Sb alloy (Fig. R7). Therefore, we deleted the TPD results in the manuscript.

Comment 9
It is suggested to add the results of CO desorption experiment.

Response
We appreciate the reviewer for this valuable suggestion. Considering the reconstruction of Sb1Cu-5 under high temperature, we alternatively conducted in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements to compare the ability of CO desorption on Cu and Sb1Cu samples at room temperature to avoid reconstruction during TPD tests. After obtaining *CO adsorbates, we suspended the applied potential and used Ar to purge into the electrolyte to sweep away *CO adsorbates ( Supplementary Fig. 26, copied below). The attenuation rate and retention time of the ATR-SEIRAS signal of *CO could reflect the ability of CO desorption [Science 350, 185-189 (2015)]. The attenuation rate was faster on Sb1Cu-5 than on Cu, manifesting its lower binding energy of *CO and better ability of CO desorption, which matched well with spectroscopy measurements and theoretical calculation results. time of *CO on Sb1Cu-5 than Cu manifested its lower binding energy of *CO and better ability of CO desorption.

Comment 10
The authors stated that "…the formation of copper oxides was attributed to oxygen susceptibility of the Cu nanocrystal surface when exposed to air".

Response
We highly appreciate the reviewer's high evaluation of our study.
As to the reviewer's concern that "Apparently there is a lower limit of Sb for the system to be selective to CO. Is there a upper limit for Sb concentration?", we demonstrated that there is an upper limit of Sb concentration for the Sb-Cu system to realize high selectivity of CO.
In new experiments, we synthesized a Cu-Sb alloy using the co-reduction method with a higher content of Sb in the precursor solution. The concentration of Sb was ca. 10 at% (denoted as Sb1Cu-10), as determined by ICP-AES. The HAADF-STEM image showed the formation of Sb aggregates in Sb1Cu-10 ( Supplementary Fig. 13, copied below) rather than even distribution of isolated Sb atoms in Sb1Cu-5. As expected, due to the formation of Sb-Cu interfaces, negligible C2+ formation and >80% FECO were found on Sb1Cu-10 ( Supplementary Fig. 24, copied below), which confirmed the role of Sb-Cu interfaces in facilitating CO desorption and limiting C-C coupling. However, more formate produced on

Response
We appreciate the reviewer's constructive comments. In this revised version of the manuscript, we have addressed all questions raised by the reviewer accordingly.
Here, we would like to emphasize the significance of this work, which distinguishes it from other works of Cu-Sb catalysts. In our work, we described isolated atomic Sb-Cu interfaces that modulated the Cu electronic structure and facilitated CO production, which was experimentally and theoretically corroborated. In contrast, other reported Cu-Sb bulk alloys, such as Cu2Sb [Nano Res. 14, 2831-2836 (2021) achieved a CO partial current density above 450 mA cm -2 with ca. 90% FECO, manifesting the excellent activity and selectivity towards CO2-to-CO on Sb1Cu interface sites. However, all the recently reported Cu-Sb catalysts failed to achieve CO partial current densities higher than 50 mA cm -2 , far from the commercially relevant scale. Therefore, the Sb1Cu-5 catalyst exhibited the best CO2RR performance among the reported Sb-Cu electrocatalysts.

Comment 1
How to define single-atom alloy catalysts? Traditionally, single-atom and alloy are two concepts. In Fig. 1  To distinguish Sb single-atom from surface Sb-Cu single-atom alloy in the catalyst, we used high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and extended X-ray absorption fine structure (EXAFS) as evidences as shown in Fig.   1. HAADF-STEM is a critical technique to identify single atoms in SAA. HAADF-STEM is based on Rutherford scattering, in which the image intensity for given atoms is roughly proportional to the square of the atomic number (Z 2 ) of the element, allowing heavy metal atoms to brightly contrast against low background supports. Since Sb is a heavier atom than Cu, we believe that all the bright spots represent isolated Sb atoms. HAADF-STEM clearly identified isolated bright spots representative of atomically dispersed Sb atoms in the Cu matrix for Sb1Cu-5 (Fig. 1b, copied below). In addition, the formation of Sb-Cu bond was also verified by Sb K-edge EXAFS results (Fig. 1d, copied below), confirming Sb and Cu are combined by alloy bonding. Considering above evidences, Sb1Cu-5 belonged to Sb-Cu single-atom alloy.

Comment 2
At present, the optimal Sb loading mentioned in the article is about 5%. Is it possible for the Sb loading to be increased significantly so that more Sb-Cu interfaces can be constructed in favor of CO2 to CO conversion?

Response
We appreciate the reviewer for raising this valuable point. To investigate the result of high Sb loading, we synthesized a Cu-Sb alloy using the co-reduction method with a higher content of Sb in the precursor solution. The concentration of Sb was ca. 10 at% (denoted as Sb1Cu-10), as determined by ICP-AES. The HAADF-STEM image showed Sb aggregation in Sb1Cu-10 ( Supplementary Fig. 13, copied below) rather than even distribution of isolated Sb atoms in Sb1Cu-5. As expected, due to the formation of Sb-Cu interfaces, negligible C2+ formation and >80% FECO were found on Sb1Cu-10 ( Supplementary Fig. 24, copied below), which confirmed the role of Sb-Cu interfaces in facilitating CO desorption and limiting C-C coupling. However, more formate produced on Sb1Cu-10 compared with Sb1Cu-5 was attributed to the formation of Sb clusters, considering that pure Sb exhibited  Supplementary Fig. 24 | CO2RR performance of the Sb1Cu-10 catalyst in a flow cell. As expected, due to the formation of Sb-Cu interfaces, negligible C2+ formation and over 80% FECO were found on Sb1Cu-10, which confirmed the role of Sb-Cu interfaces in facilitating CO desorption and limiting C-C coupling. However, more formate produced on Sb1Cu-10 compared with Sb1Cu-5 was attributed to the formation of Sb clusters, considering that pure Sb exhibited relatively higher selectivity towards formate. This result manifested the importance of isolated Sb-Cu interfaces.

Comment 3
The authors are suggested to run the CO-TPR experiment in addition to the CO2-TPR.

Response
We appreciate the reviewer for this important suggestion. We apologize for ignoring the possible reconstruction of Sb1Cu-5 under high temperature. The XRD pattern showed peaks of Sb2O3 in Sb1Cu-5 after treatment at 400℃, demonstrating phase separation of Sb and Cu metal in the Cu-Sb alloy (Fig. R7). Therefore, we deleted the TPD results in the manuscript.
We further conducted in situ ATR-SEIRAS measurements to compare the ability of CO desorption on Cu and Sb1Cu samples at room temperature to avoid reconstruction during TPD tests. After obtaining *CO adsorbates, we suspended the applied potential and used Ar to purge into the electrolyte to sweep away *CO adsorbates (Supplementary Fig. 26, copied below). The attenuation rate and retention time of the ATR-SEIRAS signal of *CO could reflect the ability of CO desorption [Science 350, 185-189 (2015)]. The attenuation rate was faster on Sb1Cu-5 than on Cu, manifesting its lower binding energy of *CO and better ability of CO desorption, which matched well with spectroscopy measurements and theoretical calculation results.

Supplementary Fig. 26 | In situ ATR-SEIRAS spectra of a) Cu and b) Sb1Cu-5 under an Ar sweep after suspension of the applied potential and c) attenuation of the *CO peak area with time.
To further confirm the better ability of CO desorption on Sb1Cu-5, we investigated the *CO retention time under an Ar sweep. The faster attenuation rate and shorter retention time of *CO on Sb1Cu-5 than Cu manifested its lower binding energy of *CO and better ability of CO desorption.

Comment 4
As Fig. 4f, Supplementary Fig. 22, and spectroscopy experimental, the CO is adsorption the top site of Cu site. And TS geometrical structure of C-C coupling is deviated the site, obviously. The authors are suggested to inset the IS and FS structures in Fig. 4d not only TS.
And the reaction activity of different Cu site which close or far Sb atom can further discuss to explain the adsorption site of CO and other intermediates.

Response
We appreciate the reviewer for this suggestion. In situ spectroscopic measurements are not contradictory to theoretical calculations. It was found by DFT calculations that the most stable adsorption for a single CO* adsorption is also atop-CO* on both Cu (211) and Sb1Cu-5 (211). However, the coadsorption of two CO* must be different from that of two single CO*-atops for CO-CO coupling (consistent with the J. Phys. Chem. Lett. 10, 533-539 (2019)). The deviated CO* adsorption is only present for CO-CO coupling, which is close to a transient state. In other words, the lifetime is relatively short. Hence, the dominant presence of CO* adsorption is still atop the site. This is not contradictory to the experimental spectra.
The IS and FS structures of CO*-CO* coupling have been added and are now shown in The Cu site nearest to the Sb atom has been discussed in our original manuscript. The Cu sites far from the Sb atom should be very similar to pure Cu. We further studied the reaction activity of the Cu site, which is the next-nearest to the Sb atom on Sb1Cu-5 (211), as shown in Supplementary Fig. 30 (copied below). As COOH* and HCOO* prefer to be adsorbed at the bridge site of Cu, the adsorption energies were close for the Cu sites, either nearest or next-nearest to the Sb atom. However, the adsorption energy of CO* at the top site on Cu (next-nearest to Sb atom, -0.21 eV) was close to pure Cu (-0.20 eV), more stable than CO* at the top site of Cu (nearest to Sb atom, -0.11 eV).
On the Cu site (next-nearest to the Sb atom), CO is also the main product due to the lower barrier compared to HCOOH and C2+ formation. However, CO* accounts for 77% of the total sites for CO2RR. This is inconsistent with the results of spectroscopic measurements, where the lower frequency of the CO* peak on Sb1Cu-5 implied weakened CO* adsorption and much lower CO coverage relative to Cu (Fig. 3d). Hence, the Cu sites (next-nearest to Sb atom) on Sb1Cu-5 should not be the main active sites. The Cu sites (nearest to Sb atom) on Sb1Cu-5 should have the major activity contributions.

Response
We appreciate the reviewer for raising this valuable point. The structures of COOH* in CO2 protonation and CO* formation are different because the water layers of these two states were different. For FS1, the proton from the water layer has been added to COOH*. It is a neutral water structure. However, for IS2, the water structure is before protonation. In other words, it is a charged water structure. The differences in water layers between FS1 and IS2 lead to the structural transformation of COOH*.

Comment 6
The obvious Stark slope of CO is obvious, why not the adsorption energy of CO* is not influenced under different potentials at Fig. 4f.

Response
Phone: +86 028-61838256 • E-mail: chuan.xia@uestc.edu.cn • Web: https://www.chuan-lab.com/ In Fig. 4f, a computational hydrogen electrode model was used to calculate the free energy change at varying potentials. The major free energy differences at varying potentials are from the electrochemical process, namely, the chemical potential variation of the electron-proton pair. The free energy changes of CO2 protonation and COOH* protonation (CO* formation) can be significantly affected by potentials, where proton transfer occurs, as shown in Fig. 4f. However, reactions without proton transfer, such as CO adsorption or desorption, are usually considered potentially independent [Nat. Nanotechnol. 16, 1386-1393(2021]. Hence, the adsorption energy of CO* is not significantly influenced under different potentials.
We have also strictly investigated the potential effects on the adsorption energy of CO*.
It also supported our explanations above. An electric field was applied in DFT calculations.
Based on a parallel-plate capacitor model, a linear correlation between the electric field and absolute potential was approximated as follows [J. Phys. Chem. C 124, 14581-14591 (2020)]: where σ is the surface charge density and ε and ε0 are the dielectric constants of vacuum and water near the interface, which were set to 8.85×10 -12 F m -1 and 2 (unitless), respectively.
The SHE could be related to the RHE by the following formula: The calculated adsorption energies of CO* changed very little (< 0.05 eV) with potentials between 0 and -1.2 V vs. RHE, which showed consistent trends for Cu (211) and Sb1Cu-5 (211). All insights and conclusions shown in the original manuscript are still reliable. In addition, by varying potentials from -0.2 to -0.7 V vs. RHE, the calculated adsorption energies of CO* weaken from -0.13 to -0.11 eV on Sb1Cu-5 (211) and from -0.23 to -0.20 eV on Cu (211), which showed consistent trends with the results of the in situ ATR-SEIRAS spectra (Fig. 3b-d).
Additional microkinetic modeling was performed to double check the reliability of the present kinetic analysis. All conclusions are not affected. Overall, the potential effects on the adsorption energy of CO* were ignored in the calculation of free energy change.
Phone: +86 028-61838256 • E-mail: chuan.xia@uestc.edu.cn • Web: https://www.chuan-lab.com/ In response to the Reviewer's comment, we have added the electric field effect on the adsorption energy of CO* in the Computational details.

Comment 7
The authors are suggested to compare the relevant work in the literature with their own.

Response
We now have compared Sb1Cu-5 with other recently reported Cu-based catalysts or non-Cu-based catalysts in Figs. 2d, 2e, Supplementary Tables 2 and 3 (copied below).
Sb1Cu-5 achieved a CO partial current density above 450 mA cm -2 with ca. 90% FECO, outperforming the previously reported state-of-the-art CO-selective electrocatalysts, manifesting the excellent activity and selectivity towards CO2-to-CO on isolated Sb-Cu interface sites. We also compared our work with other Sb-Cu catalysts (Supplementary Table 4, copied below). All the recently reported Cu-Sb catalysts failed to achieve CO partial current densities higher than 50 mA cm -2 , far from the commercially relevant scale.

Comment 8
Some other minor points: Check the typos throughout the manuscript, e.g., Line 107, NaHB4.
The label of Figure S20 is messed up, a is Cu, b is Sb-Cu?
Check the format of references, especially journal abbreviations.
The Sb1Cu-5, and the Sb1Cu-1.5, What does subscript 1 mean? We appreciate the reviewer for pointing out these points. We are sorry that we made some mistakes in the manuscript and supplementary information. We have corrected "NaHB4" to "NaBH4" in line 107 and checked the typos throughout the manuscript. The labels in Supplementary Fig. 20 were messed up. Figure a shows the H-cell performance on the Sb1Cu-5 catalyst, while Figure b shows the H-cell performance on the Cu catalyst. The corrected figure is copied below. In addition, we have changed the format of references to the standard format of Nature Communications. When we describe the SAA catalyst, we use A1B to indicate that A atoms are isolated and dispersed in the B metal host (e.g., Pb1Cu [Nat. Nanotechnol. 16, 1386-1393 (2021)], Pd1Ni [Nat. Commun. 10, 4998 (2019)]). Here, the subscript "1" means that Sb atoms are isolated and dispersed in the Cu host, forming atomic Sb-Cu interfaces.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): Thanks the authors for taking into consideration of all my comments. The modifications are sufficient and reasonable. I can now suggest it for publication on Nature Commination.
Reviewer #3 (Remarks to the Author): I have checked the revised manuscript and the author's response. I think the authors have made large effort to response all the comments and questions. In the responses, the authors compares the catalyst performance made by themselves with the reported CuSb, highlighting the innovation of the article. Most reports on the performance of CuSb catalysts are carried out in H-type electrolyzer. The authors need to quote the relevant CuSb articles reasonably in the manuscript, and compare the performance of catalysts under the same experimental conditions. I think the revised manuscript should be proper for publication after the authors further improved it.

Response
We highly appreciate the reviewer for time and insightful comments on our work.

Response
We appreciate the reviewer's valuable suggestions. We have cited the relevant Cu-Sb articles in the manuscript (ref. 37-39). We have also compared the CO2RR performance of Sb1Cu-5 with recently reported Cu-Sb catalysts in an H-cell (Supplementary Table 4, copied below). In our work, Sb1Cu-5 achieved above 90% FECO at a current density of -10 mA cm -2 with the stability of 100 hours in an H-cell. Both the CO partial current density and FECO of Sb1Cu-5 are higher than those of reported Cu-Sb catalysts (e.g., Cu2Sb NA/CF and Sb modified Cu). Because of different mass loading in the reported Cu-Sb articles, we also compared their mass activity normalized by the catalyst mass. Sb1Cu-5 exhibited the highest mass activity among the reported Cu-Sb catalysts. In addition, the durability of Sb1Cu-5 was much longer than other Cu-Sb catalysts. Therefore, the Sb1Cu-5 catalyst exhibited the best CO2RR performance among the reported Sb-Cu electrocatalysts.